The present invention relates to a probe holder suitable for low current measurements.
Typically, in the construction of a probe card, a dielectric board is used as a base. A plurality of probing devices are mounted in a radial arrangement about an opening in the board so that the probing elements of these devices, which may, for example comprise slender conductive needles, terminate below the opening in a pattern suitable for probing the contact sites of the test device. The probing devices are individually connected to the respective channels of a test instrument by a plurality of interconnecting lines, where the portion of each line that extends between the corresponding probing device and the outer edge of the dielectric board may comprise an interconnecting cable or a conductive trace pattern formed directly on the board. In one conventional type of setup where the test devices are integrated circuits formed on a semiconductor wafer, the probe card is mounted by a supporting rig or test head above the wafer, and a support beneath the wafer moves the wafer so that each device thereon is consecutively brought into contact with the needles or probing elements of the probe card.
With particular regard to probe cards that are specially adapted for use in measuring ultra-low current (down to the femtoamp region or lower), probe card designers have been concerned with developing techniques for eliminating or at least reducing the effects of leakage currents, which are unwanted currents that can flow into a particular cable or channel from surrounding cables or channels so as to distort the current measured in that particular cable or channel. For a given potential difference between two spaced apart conductors, the amount of leakage current that will flow between them will vary depending upon the volume resistivity of the insulating material that separates the conductors, that is, if a relatively lower-resistance insulator is used, this will result in a relatively higher leakage current. Thus, a designer of low-current probe cards will normally avoid the use of rubber-insulated single-core wires on a glass-epoxy board since rubber and glass-epoxy materials are known to be relatively low-resistance insulators through which relatively large leakage currents can flow.
One technique that has been used for suppressing interchannel leakage currents is surrounding the inner core of each lead-in wire with a cylindrical “guard” conductor, where the “guard” conductor is maintained at the same potential as the inner core by a feedback circuit in the output channel of the test instrument. Because the voltage potentials of the outer guard conductor and the inner conductive core are made to substantially track each other, negligible leakage current will flow across the inner dielectric that separates these conductors regardless of whether the inner dielectric is made of a low- or high-resistivity material. Although leakage current can still flow between the guard conductors of the respective cables, this is typically not a problem because these guard conductors, unlike the inner conductive cores, are at low impedance. By using this guarding technique, significant improvement may be realized in the low-level current measuring capability of certain probe card designs.
To further improve low-current measurement capability, probe cards have been constructed so as to minimize leakage current between the individual probing devices which mount the probing needles or other elements. With respect to these devices, higher-resistance insulating materials have been substituted for lower resistance materials and additional conductive surfaces have been arranged about each device in order to perform a guarding function in relation thereto. In one type of assembly, for example, each probing device is constructed using a thin blade of ceramic material, which is a material known to have a relatively high volume resistivity. An elongate conductive trace is provided on one side of the blade to form the signal line and a backplane conductive surface is provided on the other side of the blade for guarding purposes. The probing element of this device is formed by a slender conductive needle, such as of tungsten, which extends in a cantilevered manner away from the signal trace. Such devices are commercially available, for example, from Cerprobe Corporation based in Tempe, Ariz. During assembly of the probe card, the ceramic blades are edge-mounted in a radial arrangement about the opening in the card so that the needles terminate within the opening in a pattern suitable for probing the test device. The conductive backplane on each blade is connected to the guard conductor of the corresponding cable and also the corresponding conductive pad or “land” adjacent the opening in the card. In this manner each conductive path is guarded by the backplane conductor on the opposite side of the blade and by the conductive land beneath it.
It has been found, however, that even with the use of guarded cables and ceramic probing devices of the type just described, the level of undesired background current is still not sufficiently reduced as to match the capabilities of the latest generation of commercially available test instruments, which instruments are able to monitor currents down to one femtoamp or less. Thus, it was evident that other changes in probe card design were needed in order to keep up with the technology of the latest test instrument design.
However, in the design of such probe cards the ceramic blades are permanently mounted to the probe card and thus when damaged the entire probe card may need to be replaced or the damaged ceramic blade somehow repaired at substantial expense and effort. Referring to FIG. 1, in order to provide probe tips that are more easily replaced, a probe housing 10 with a replaceable probe tip 12 was designed. A pair of triaxial cables (not shown), each of which includes a shield, a guard, and a signal conductor, extend from measurement equipment (not shown) to a location within a chamber (not shown) that encloses the probe tip 12, the probe housing 10, and the test device. Each triaxial cable is connected to a respective coaxial cable 14 and 16 that includes a guard and a signal conductor. The shield conductor of each of the triaxial cables may be connected to the chamber, if desired. The chamber environment is shielded so it is unnecessary to include the shield conductors all the way to the probe housing 10. In addition, the probe housing 10 includes relatively small connectors which are much more suitable for connection to relatively small coaxial cables 14 and 16, as opposed to relatively large triaxial cables. The probe housing 10 includes a pair of connectors 18 and 20, each of which provides a connection to a respective one of the coaxial cables 14 and 16. The guard of each of the coaxial cables 14 and 16 is electrically connected to the conductive exterior of the probe housing 10, which reduces the capacitance and leakage currents to the probe tip 12.
Ideally in a two lead coaxial cable system a “true Kelvin” connection is constructed, although not shown in FIG. 1. This involves using what is generally referred to as a force signal and a sense signal. The signal conductor from one of the coaxial cables is considered the force conductor, while the signal conductor from the other coaxial cable is considered the sense conductor. The force conductor is brought into contact with a test pad on the wafer. The force conductor is a low impedance connection, so a current is forced through the force conductor for testing purposes. The sense conductor is a high impedance connection and is also brought into contact with the same test pad on the wafer, preferably in close proximity to the sense conductor, in order to sense the voltage. As such the current versus voltage characteristics of the test device can be obtained using the force and sense conductors.
To calibrate the “true Kelvin” connection, first an open circuit test is performed to measure the capacitance without the test pad capacitance. This is performed by picking up the probe and shorting the probe tips of the sense and force conductors together with both suspended in air. The open circuit test is difficult to perform. Second, a short circuit test is performed to measure the capacitance when the force and sense conductor tips are on the test pad. From the open circuit test and the short circuit test the cable impedance is obtained and thereafter used for offsetting during subsequent measurements. Unfortunately, calibration of a “true Kelvin” connection is difficult and time consuming to perform. Additionally, the current flowing through the force conductor is generally known but the resistance drop along the length of force conductor results in the exact voltage at its end to be unknown, therefore the measurement can be inaccurate. Further, the test pads on the test device are normally small, in order to minimize cost, which makes it difficult to position two needles on the test pad. Furthermore, using two needles requires additional space for the needles and supporting structure that may not be available when a large number of probe needles are simultaneously necessary to test a small area of the test device, such as a silicon wafer.
Referring again to FIG. 1, to permit the use of a single probe tip, which permits more tests to be simultaneously performed in a confined area, the force conductor 22 and the sense conductor 24 are electrically connected together with a combined conductor 26 within the probe housing 10. Coaxial cable 14 would be the force connection while cable 16 would be the sense connection. The guard conductor of the force cable 14 and the guard conductor of the sense cable 16 are electrically connected to the conductive probe housing 10. The combined force and sense conductor 26 is electrically connected to a probe connector 28 at one end of the probe housing 10. A rigid coaxial probe tip cable 30 is detachably connected to the probe connector 28. The rigid coaxial probe tip cable 30 includes both a copper guard conductor 32 plated with gold and a central signal conductor 34 made of tungsten. The guard conductor 32 of the rigid coaxial probe tip cable 30 is electrically connected to the probe housing 10, which is in turn connected to the guard conductors of the coaxial cables 14 and 16. The length of the signal path extending from the point that the force conductor 22 and sense conductor 24 are connected together carries current during measurements which results in a voltage drop from any internal resistance in that portion of the signal path. The assumption is that for low current applications, the voltage drop due to the resistance is small because the junction is close to the probe tip 12 and the conductor has low resistance. However, the rigid coaxial probe tip cable 30 is difficult to replace if defective or damaged during use. The test device shown in FIG. 1, provides reasonably accurate low current measurements. Unfortunately, it was observed that the device shown in FIG. 1, when used over a wide range of temperatures, such as −65 degrees celsius to 300 degree celsius, results in unacceptable levels of noise. For example, in one commercial embodiment, noise in the range of +−100 femtoamps was observed over only a temperature range from room temperature to 150 degrees celsius. As previously mentioned, modern measurement instruments are capable of measuring much lower current levels and thus such noise levels obscures low current measurement levels under 10 femtoamps.
What is desired, therefore, is a low current measurement device that has substantially lower noise levels. In addition, such a measurement device should be provide for easy replacement of probe tips.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.